In the simplest concept a two dimensional digitizer consists of a movable cursor coil and a system of grid wires so arranged that the voltage induced in the grid wires by the cursor (or vice versa) is a function of the position of the cursor coil relative to the grid wires.
In most systems the grid wires are connected to form a single continuous convoluted winding with the induced voltage being produced between the two ends of the winding.
The grid wires are usually mounted just beneath or contained within a flat surface which is referred to as a digitizing tablet. The graphical material to be digitized is placed upon the surface of the tablet and the cursor coil is positioned as required to digitize the information. The process of digitizing involves producing two electrical signals (X and Y), which possess a measurable property (amplitude, frequency or pulse width), which is in known relationship to the XY co-ordinates of the cursor position with reference to some pre-defined cartesian co-ordinate system.
In most practical implementation the X and Y co-ordinates are obtained using two identical othogonal systems. The system is ideally only sensitive to changes in position along a single axis arranged to be the X axis for one system and Y axis for the other.
In practice, it is impossible to produce two completely independent orthogonal systems because of edge effects which manifest themselves as the cursor coil approaches the edge of the tablet. These errors can be minimized by restricting the cursor from operating near the tablet edge and/or providing error compensation (soft ware or hard ware) for the edge effect.
For example, a compensating signal can be introduced into the X co-ordinate signal based upon the Y co-ordinate information (which signifies how close the cursor is to the edges of the tablet which run parallel to the X axis).
An extensive discussion of an automatic coordinate determining device of the type involved in the present application appears in U.S. Pat. Nos. 3,647,963 and 3,801,733. Discussion of a modified form of coordinate determining device in which a slowly varying difference signal is employed appears in U.S. Pat. No. 3,735,044. The technical description contained in these patents is incorporated into this application as background information to the same extent as if fully set forth herein.
In a typical position-measuring device of the digitizer type, the alternating current excitation signal applied to the cursor is preferably 3 kHz, and it may be assumed for the purposes of the present application that 3 kHz is the frequency of the excitation signal applied to the cursor.